We expected Lucy to go undercover but not this soon.

Lucy's interest in undercover work dominated The Rookie Season 5 Episode 20.

Isabel previewed the consequences of that choice, but that didn't deter Lucy from pursuing it.

Elsewhere, Laura's relationship with Mark has its ups and down because they live in different time zones, but when it's good, it's perfect.

Spoilers for The Rookie Season 5 Episode 21 tease an action-filled two hours as LAPD and Feds team up to rescue one of their own while Lucy goes undercover.

The kids have grown, everybody!

Nolan has settled into his desired career path as a training agent, and even though Lucy was slow to decide what she wanted to do with her career, she is now getting into the groove.

It emerged after a few incidental operations that undercover work was the path for her, and after wrestling with that choice for a while, she appeared to have made her decision.

Going undercover can be mentally and physically challenging because you have to be alert every second, and as Isabel advised, it can be unhealthy.

While it is clear that Lucy has what it takes to pull off an undercover mission, it may be challenging.

Her relationship with Tim is the biggest concern in all of this. Tim has lived through a marriage with an undercover agent, and the results were not good.

His ex-wife became a mentally unstable addict, ultimately leading to their marriage breakdown.

When Lucy revealed she was thinking of going undercover, Tim had a mixed reaction. He might have hated the idea and didn't speak about it. Or, he might have been concerned.

Lucy goes undercover to infiltrate an illegal firearm operation, and if there's one thing clear about gun dealers, they are trigger-happy. That makes an already dangerous situation more dangerous.

This operation will be the true test of Tim's feelings about Lucy's undercover work. They have been undercover together as Dim and Juicy, but he was present in case things went south, so he felt he had little control of the situation.

This time around, not at all.

To help Skip Tracer Randy, Nyla agreed to take one of those hard-to-solve cases from a detective. These cases involve the presence of human remains without any way of identifying the deceased.

Such a case finds Nyla and Nolan when severed limbs appear all over the city, and they must unravel the mystery.

The Rookie Season 5 Episode 20 lacked much action, and they found a way to remedy that in this episode.

We will get another crossover with the Feds.

The case about severed limbs crosses to other states, and as such, it goes beyond Nyla and Nolan's jurisdiction. It gets kicked up to the Feds, whose specialty is solving such cases. And Garza's team takes over.

Antoinette and Elena team up, and using some forensic magic, they find a breakthrough in the case allowing the team to pursue a lead.

Meanwhile, Mark Atlas visits and he and Laura hang out with Brendon and Antoinette. That's something that Brendon would consider a double date, and if he voices that opinion, Laura will shoot it down.

Laura and Mark have a complicated relationship. At its best, there is some action and flowers; at its worst, there is arguing and threats of breaking up, which has happened before.

The relationship's biggest issue is that they live on opposite coasts.

Things had been getting more serious over the past while, especially after Valentine's, and the need and desire started affecting the mind at some point.

Laura is usually the shots caller in the relationship and is never afraid to voice discontent when needed.

During this visit, what was supposed to be a great night of chilling with friends and co-workers goes south when Mark and Laura get into it.

To prevent the situation from further escalating, Laura decides to drive it off and shoots down Mark's attempts to accompany her.

The attempts to catch the killer leaving limbs around seem to have worked because he gets spooked and retaliates.

As Laura tries to get into her car after the altercation with Mark, she is kidnapped by the limbs killer.

It is a race against time to find out why the killer took her, find him, find where Laura is, and get to her in time before an even bigger tragedy than a kidnapping occurs.

It is all hands on deck as the FBI enlists Tim, Nyla, Nolan, Sgt Grey, and anyone else available to catch the killer before he adds Laura to his list of successful conquests.

This can be expected to be an intense action-driven episode with the life of an FBI agent in jeopardy and Lucy getting into the middle of a gun den.

It's doubtful there will be that volley of competitive humor between Tim and Laura because the situation won't allow it.

What do you think about how it will all play out based on these spoilers?

Check out the promo below.

How excited are you for this crossover?

Let us know in the comments section, and remember to watch The Rookie online to catch up on the season before the crossover and the season finale.

Catch the episode live on ABC on Tuesday, April 25 at 9 p.m. and a review right after here on TV Fanatic.

You can check out our episodic roundtable discussions to get a different perspective on the events in an episode.

Denis Kimathi was a staff writer for TV Fanatic. He has watched more dramas and comedies than he cares to remember. Catch him on social media obsessing over [excellent] past, current, and upcoming shows or going off about the politics of representation on TV. Follow him on X.