When TV shows collide in crossover episodes, it can result in some of the most unforgettable moments that forever change both shows. A TV crossover sees the characters of one show intermingling with those from another, often connecting plots, themes, and writing styles in the process. Sometimes, crossover episodes link shows from the same universe, setting up spinoffs or reuniting former cast mates. In other instances, crossovers link two or more unexpected shows together to produce a dynamic television episode.

A good crossover can keep a show alive and add new dimensions to its already-existing story. The writing can bring out new facets of a character or give a TV universe a wider sense of reality. While many crossovers are throwaway one-shots, some have huge impacts on their respective shows, forever changing the characters and plots.

10 I Love Lucy And The Adventures Of Superman

I Love Lucy Season 6, Episode 13

Superman's appearance in I Love Lucy is considered the first sitcom crossover episode. In an I Love Lucy installment titled “Lucy and Superman,” Lucy (Lucille Ball) tries to make Little Ricky’s (Keith Thibodeaux) birthday party outshine everyone else's. Lucy begs Ricky (Desi Arrnaz) to use his connections to get George Reeves to attend the party as Superman. When Reeves can't make it, Lucy dresses up as Superman herself.

In true I Love Lucy fashion, Lucy's grand entrance — which involves standing out on the ledge of their third-floor apartment and leaping in through the window — goes terribly wrong. Fortunately, Superman actor George Reeves sweeps in to save the sitcom's heroine. Reeves is not credited for his role in this 1957 sitcom episode, as Ball didn't want children to know Superman wasn't real. Still, this was a game-changer at the time it aired, and it's a fun chapter of the show to look back on.

9 Grey's Anatomy And Station 19

Grey's Anatomy Season 17, Episode 7 | Station 19 Season 4, Episode 6

Grey's Anatomy frequently crosses over with spinoff Station 19, but the death of Andrew DeLuca (Giacomo Gianniotti) occurs in one of the most significant crossover episodes. Grey's Anatomy fans were not given the closure of learning how DeLuca died in season 17, episode 7. Instead, they had to tune in to Station 19 season 4, episode 6 for further details. In the crossover episode with the fellow Shondaland show, it's revealed that the doctor was brutally stabbed while in pursuit of a sex trafficker.

In Station 19 season 4, DeLuca tracks down a sex trafficker named Opal (Stephanie Kurtzuba). At the end of the episode, a man bumps into DeLuca and knocks him to the ground. Grey's Anatomy shows his tragic fate, as he is rushed to Grey Sloan Memorial. His absence is felt in both series, and it's one of the ways Grey's Anatomy and Station 19 crossovers have changed both shows forever.

8 Chicago Med, Chicago Fire, And Chicago P.D.

Chicago Fire Season 8, Episode 4 | Chicago Med Season 5, Episode 4 | Chicago P.D. Season 7, Episode 4

NBC's Chicago franchise has staged crossovers before, but none were as ambitious as its three-part "Infection." This crossover sees characters from Chicago Fire, Chicago Med, and Chicago P.D. working together to deal with flesh-eating bacteria. This three-hour crossover blends genres, as Chicago Med is a soap, Chicago Fire is a lighter drama, and Chicago P.D. is a procedural.

It starts when Matthew Casey (Jesse Spencer) sees a man collapsing during a Chicago Bears game. Upon closer inspection, it's revealed that the guy has a flesh-eating infection on his leg. His last words are "BRT." While responding to a university fire, the team notices the same letters written on the wall. This leads to an investigation, which causes a massive narrative to unravel — one that changes everything by showing what all three shows are capable of when they come together.

7 Scandal And How To Get Away With Murder

Scandal Season 7, Episode 12 | How To Get Away With Murder Season 4, Episode 13

The crossover event between How to Get Away with Murder and Scandal brings together two of the most powerful women on primetime TV. How To Get Away With Murder's Annalise Keating (Viola Davis) teams up with Scandal's Olivia Pope (Kerry Washington) to work on a class-action suit regarding the mass incarceration of Black people in America. The fast-tracking reform comes soon after Olivia's fall from political grace, making it more challenging for Annalise.

In the first part of the crossover event, Olivia, who is now teaching, writes "How to Survive a Scandal" on the chalkboard, echoing the first episode of How To Get Away With Murder. The crossover episode celebrates the differences between the two ambitious women, with the pair not afraid to criticize the other's flaws. Their conversation in the beauty shop feels quietly revolutionary, considering the responsibility of wealthy black women.

6 Ally McBeal And The Practice

Ally McBeal Season 1, Episode 20 | The Practice Season 2, Episode 26

David E. Kelly's two legal series share a murder case in this 1997 crossover. Ally McBeal was on Fox, and The Practice was on ABC, yet the two rival networks came together to help out their star writer. With one show having a more comedic tone and the other having a darker one, the plot somehow manages to find elements that complement both styles. Ally McBeal's ratings were struggling at this point, so the show might not have had further seasons without this crossover.

In this crossover, Ally McBeal's (Calista Flockhart) firm handles its first criminal case involving Maria Hanson, a woman accused of murdering her husband with an ax. Because the lawyers at her firm, Fish and Cage, have mixed feelings about accepting a criminal case, they approach Bobby Donnell (Dylan McDermott), whose practice has a reputation for excelling in criminal law. It's a seamless way to blend two Boston-based shows and was one of the first successful multi-network crossovers.

5 Homicide: Life On The Street And Law & Order

Law & Order Season 6, Episode 13

Law & Order has produced three two-part crossovers with Homicide: Life on the Street. A case would begin with Law & Order in New York City for part 1 before moving the action to Baltimore for part 2. One of the most significant of the crossovers in season 6's "Charm City" sees the first appearance of Richard Belzer's Detective John Munch in the Law & Order world.

In this crossover episode, Detectives Pembleton (Andre Braugher) and Bayliss (Kyle Secor) clash with Briscoe (Jerry Orbach) and Curtis (Benjamin Bratt) when they come to New York to investigate a subway explosion. Upon the cancellation of Homicide: Life On The Street in 1999, Richard Belzer was offered a regular slot as John Munch on Special Victims Unit. Munch appeared in a total of 10 series across five networks since his debut in 1993.

4 Law & Order: SVU And Law & Order: Organized Crime

Law & Order: SVU Season 22, Episode 9 | Law & Order: Organized Crime Season 1, Episode 1

Elliot Stabler’s (Christopher Meloni) return to the Law & Orderverse was a two-hour event that kicked off the Organized Crime spinoff. Both the SVU episode and Organized Crime's opening episode follow the murder of Detective Elliot Stabler's wife. This is Stabler's first appearance in the NBC drama after a decade, with Meloni having left the show in season 12.

After reuniting Stabler with his longtime partner, Detective Olivia Benson (Mariska Hargitay), the show finally explains what happened to him. SVU reveals Stabler was living in Italy with his family until his wife's murder. Stabler's character in Law & Order: Organized Crime seeks to rebuild his life after his devastating loss, joining an elite task force that is looking to take down the city's powerful criminal syndicates one by one.

3 Angel And Buffy The Vampire Slayer

Angel Season 1, Episode 8

Buffy makes her appearance only eight episodes into Angel's debut spinoff season. After a fight with a demon turns Angel into a human, his ex-girlfriend Buffy comes to visit. The pair bask in the glow of being able to truly be together. As quickly as they are reunited, they are torn apart when Angel sacrifices his humanity to protect Buffy.

Unable to live with the knowledge of what a relationship with Buffy would have been like, Angel visits the Powers That Be and asks them to erase the day. As Buffy kisses Angel goodbye, she promises to always remember their limited time together. She ultimately forgets their one perfect day, while Angel is stuck remembering it forever. Angel and Buffy The Vampire Slayer frequently crossed over, but no moment was as powerful as this season 1 heartbreak.

2 Supergirl, Arrow, The Flash, And Legends Of Tomorrow

Supergirl Season 3, Episode 8 | Arrow Season 6, Episode 8 | The Flash Season 4, Episode 8 | Legends of Tomorrow Season 3, Episode 8

In December 2019, The CW launched the ambitious crossover event Crisis On Infinite Earths, which spanned several of the network's Arrowverse shows. Not only did heroes from the Arrowverse get involved in Crisis on Infinite Earths, but the crossover brought in Smallville's Clark Kent (Tom Welling), Lucifer (Tom Ellis), Brandon Routh's Superman, and Ezra Miller's Flash. This crossover was the first time the Arrowverse and the DCEU acknowledged each other. The scope of this crossover was practically unprecedented for television, delivering Avengers-level teamwork on the small screen.

1 The Flash, Supergirl, Legends Of Tomorrow, And Arrow

The Flash Season 3, Episode 8 | Supergirl Season 2, Episode 8 | Arrow Season 5, Episode 8 | DC’s Legends of Tomorrow Season 2, Episode 7

Inspired by the 1989 DC Comics event, Invasion! was an ambitious crossover that brings Flash, Arrow, and the Legends of Tomorrow together. This TV event also saw Supergirl heading to Earth-1 after her TV show jumped from CBS to The CW. The TV event was part of Arrow's 100th episode celebration and saw deceased characters like Laurel Lance and Moira Queen returning via a dream reality.

This TV crossover found Supergirl interacting with her fellow superheroes for the first time and also saw Brandon Routh returning as Superman. Invasion! also marked the first appearance of the abandoned STAR Labs hangar. From addressing the repercussions of Barry's recent timeline meddling to the epic fights with Dominators, "Invasion!" was the first of its kind in terms of proportion and storytelling. Unlike its predecessors, this crossover didn’t have to worry about setting up a new show, allowing it freedom with its cast and plot.