As Castle is wont to do after emotionally draining episodes, like the last couple weeks of 3XK turmoil, this week they slow it down with a light-hearted and fun episode. This week Caskett and team head to “Mars” to solve a mystery.  Enjoy!

***** SPOILERS BELOW *****

Castle and Beckett come home, expecting to have a quiet night, and instead they find Alexis and a friend playing laser tag.  Castle gets a little hung up on Alexis playing laser tag without him, when Martha and a male companion come downstairs.  It seems like the loft is getting a little crowded!  They get called away to a murder on “Mars.”  Except, it’s not really Mars, it’s a recreated environment by Viggo Jansen (David Clayton Rogers), who is conducting a simulation in advance of a hopeful, future Mars mission.

Their murder victim is one of the simulation’s astronauts, Tom Richwood (Yves Bright).  Castle proclaims this is the “best case ever” when he is told he has to wear a spacesuit to go down to the simulation, because the air contains chemicals to simulate Mars-like conditions.  The simulation supervisors insist that no one could have gotten in or out of the simulation area, so they’re thinking it must have been one of the other astronauts who killed him.  There is a computer system onboard the shuttle named “MIRA,” which Castle says is a little 2001.

Beckett tells Jansen she wants to remove the astronauts from the simulation and take them to the precinct to interrogate them.  He doesn’t want to shut everything down and lose all of their progress on the simulation.  They agree to work together to find out who killed Tom, but for now they’re going to continue the simulation and allow Beckett’s team to have access to the simulation to do what they need to do.  The team finds a video Tom took, where he claims to be seeing things around the simulation.

The security system ends up having been hacked, meaning someone could have been getting in and out of the simulation unseen.  The breach traces back to applicant Clint Granger, who had been kicked out of the program.  The guy has a solid alibi though, so it looks like it isn’t him.  Castle and Beckett explore an abandoned utility access area that runs under the simulation, and discover a ventilation tunnel that someone had been able to access.  Beckett finds the murder weapon in the tunnel, but they get interrupted when they start hearing noises of something around them.  Whoever it was is gone, but they find a gas mask that they dropped on their way out.

Back home, Castle and Beckett’s second attempt at a quiet night gets ruined again by a full house, and Castle says he is going to call a family meeting.

The gas mask traces back to Granger.  He says he didn’t kill Tom, but he was working with someone else from the simulation to steal materials.  It turns out Granger was hired by one of Jansen’s competitors, Sir Ian Rasher (Maxwell Caulfield), to help him get a competitive advantage.  Granger says another astronaut in the simulation was dropping off materials to him, and beat the system by cloning his tracker before they planted it in his skin, so he could move around unsuspected.

Beckett interrogates the astronaut who says it wasn’t him, and although he was unaccounted for at the time of the murder, he was taking pictures and downloading schematics.  He has proof that he was elsewhere when the murder occurred.  He also tells Beckett that although he didn’t kill Tom, no one liked him.  They take a look at the pictures and discover the rover was going towards Tom with the murder weapon in its grasp.  MIRA must have commanded it to do so, but MIRA can’t think on her own, so someone told her to kill Tom.  Jansen goes with Beckett and Castle to ask MIRA what happened.

MIRA goes crazy sci-fi movie on them and its clear she has been corrupted, when they try to reset her she releases noxious gases around them and it gets sketchy for a minute.  Beckett remembers they aren’t actually on Mars and calls for help.  Whoever corrupted MIRA convinced her that Tom was a threat to the mission, so Jansen says it has to be one of the astronauts.  Castle, as usual, comes up with a way to get answers.  They gather the three remaining astronauts and throw out the theory that all three of them were in on it together.

All three of them had been able to alibi each other from the beginning, making it what seemed like the perfect crime.  The astronauts were afraid the team couldn’t survive with Tom on it, since all of them disliked him, so in their minds he was in fact a threat to the mission.  Castle lies and says they have back up information from MIRA’s memory in a black box and the astronauts cave and admit their involvement.

Back at the loft, Martha tells Beckett and Castle they deserve space at the loft and she’s going to be moving out.  Looks like the family talk wasn’t necessary, and I’m sure Martha will still come up with ways to be around and see the family!

What did you think of the episode?  Comment below!

Castle airs on Mondays at 10/9c on ABC. Watch full episodes on ABC.Go.com, Hulu and Amazon Instant Video.