Invading Castle Evergrim is the fist event that happens in Grim Tales: What About Mimi. Mimi tries to invade Castle of Grim to search an artifact for her father.
Him has recaptured Mimi and killed JeffJeff for their betrayals. In order to spare the deceased spider's soul Mimi must break into Castle of Grim's vault and steal a specific item there in which will serve as a replacement for Him's severed claw.
Mimi begins her infiltration by climbing the mountainside that surrounds the castle. Lord Pain spots her on security camera's and sends guards to dispatch her, who are instead easily defeated by Mimi.
Junior's best efforts to negotiate and calm Mimi only gets his head ripped off, followed by a retaliation from Minnie that only results in a counter attack that pins Junior on top of her (something that Minnie is alright with).
Mimi's efforts into the vault are halted by the appearance of Mandy's guard dog Cerberus.
Meanwhile Mandy is alerted to the break in by Pain, but only becomes concerned after she learns that Mimi's heading for the vault. Deciding to deal with the situation in person she finishes her business with Dan Phantom.
The two have just finished conceiving a child, a process both immensely enjoyed, with the intention of raising him or her to continue Dan's legacy of senseless death and destruction. To this end Mandy informs Dan he is welcomed to any of the food in the castle, but by the time the sun comes up he will no longer be welcomed in the castle.
The two go their separate ways after that, Dan to the kitchen where he discusses the child he just conceived with Grim, while Mandy goes straight to the vault. There she discovers her guards defeated and Mimi eyeing her old Tricycle. Although Mandy takes the opportunity to congratulate Mimi on her progress but claims that she still must take her severed head.
The two enter a ranged fight, Mimi landing a blow that forces Mandy to remove her housecoat. Grim comes in with Mimi's map explaining her target prompting Mandy to give the deviling what she's looking for: Horror's Hand. After witnessing the Battle in Megaville Mimi is incapacitated by the Pentagram Generators and her life spared thanks to Junior's intervention.
Mimi becomes the "pet" of Junior, Mandy is now pregnant with the child/ren of Dan Phantom and Junior's relationship with his sister worsens dramatically, thanks to Mimi's presence. The invasion of the castle serves as the last straw for Mandy who is currently preparing for war against Mimi's father HIM.