Under Lock and Key revolves around an intense quest to stop the powerful mutant Apocalypse from rising. The X-Men and their long-time adversaries, the Brotherhood of Mutants, must put aside their differences and collaborate to prevent this ancient menace from unleashing chaos upon the world. This episode stands out for its choice to highlight the original X-Men line-up, giving fans a delightful nod to the roots of the franchise.

The plot kicks off with a high-stakes mission involving several key characters, including Cyclops, Jean Grey, Beast, Iceman, and Angel — all members of the original X-Men team. These characters must track down and secure ancient artifacts that could be used to unlock Apocalypse’s tomb. The episode expertly weaves action and suspense with moments of camaraderie and tension, showcasing the enduring appeal of these classic heroes. The inclusion of the original roster not only adds a layer of nostalgia but also emphasizes the timeless nature of their heroism and teamwork.

One of the central themes of Under Lock and Key is the idea of unity in the face of a common threat. The episode underscores the importance of setting aside personal differences and historical grievances to achieve a greater good. It’s a powerful reminder that even long-standing rivals like the X-Men and the Brotherhood can find common ground when the stakes are high enough. And by the end of this season, things are going to be very serious indeed.

Gambit breaks into Angel‘s house to steal an ancient relic. Angel catches him and chases him outside, where Gambit hands the relic to Mesmero. Angel confronts them, but Mesmero uses his powers to knock Angel out. Magneto then appears, traps Mesmero, and takes the relic, revealing it to be one-half of the Spider stone, a key to releasing the powerful mutant En Sabah Nur, also known as Apocalypse.

At the Institute, the X-kids are playing a game of Mutant ball when Angel arrives and informs them of the situation. Meanwhile, Magneto’s base is abuzz with activity as Mastermind searches Mesmero’s mind for answers. Magneto learns that the Spider stone is crucial to releasing Apocalypse and decides to stop Mesmero from obtaining the other half of the stone. The X-Men, led by Xavier, also head to London to prevent Mesmero from completing his mission.

In London, the X-Men and Magneto’s Acolytes converge at a museum where the other half of the Spider stone is kept. A battle ensues, and Mesmero loses the Spider stone, which merges with the other half to form a giant mystic spider. Magneto tries to stop the spider but struggles, and Mesmero escapes. The spider bursts out of the museum, causing chaos in the city. Magneto orders his crew to destroy the spider, but Xavier insists on capturing and protecting it, believing it to be a guardian rather than a key.

Despite their efforts, the X-Men and Magneto’s Acolytes are unable to stop the spider. Gambit crashes a double-decker bus into it, slowing it down just enough for Magneto to catch up. Magneto uses his full power to destroy the spider, but Xavier reveals that this was exactly what Mesmero wanted. The spider was a guardian created to prevent Apocalypse from escaping, and its destruction means the Second Door will open, bringing them closer to Apocalypse’s release. Xavier emphasises the urgency of finding out where the spider was heading. If Mesmero finds the third key and releases Apocalypse, the world will change forever…

The Warren Worthington III of the previous X-Men series was also an antique collector, as seen in Beyond Good and Evil. He was also targeted by a thief in that episode, namely Psylocke.

Baseball is a traditional X-Men/New Mutant past time. The ‘no powers’ rule that Beast invokes (and gives up with) usually doesn’t get used – or adhered – to.

Mastermind, Magneto and Xavier speak as if they know of Apocalypse, or at least have heard of his existence. It’s enough to scare the heebie-jeebies out of Erik!

In a nice touch, Kurt has embraced his brotherly side in regards to Rogue, now that Mystique’s adoption of her has been revealed. Doesn’t mean Rogue is happy about it though.

London has been the site of more than one battle in the X-Men animated series, both times involving Apocalypse, in Come the Apocalypse and Beyond Good and Evil.

The team that Xavier gathers for the mission in this episode, including stowaway Bobby, are the original line-up from the X-Men‘s first comic appearance in 1963. Bobby ices up for the first time (also see X-Men: The Last Stand). Bobby effectively joins the main team as of this episode.

The way Bobby and Hank enter the museum is the same way they enter Xavier’s study in X-Men #1.

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