'Hawaii Five-0' Season 4 Episode 20 Spoilers: Team Investigates Radical Students

Danny and McGarrett got caught in a major explosion on the latest episode of "Hawaii Five-0."

In "Ku l Ka Pili Koko," (Blood Brothers), the Five-0 and SWAT teams go to a location where an illegal weapon exchange is supposedly taking place. The teams find nothing, and decide to leave.

Just as Danny (Scott Caan) and McGarrett (Alex O'Loughlin) are about to leave, McGarrett hears a ringing sound. The pair open a back door at the parking garage and find a man gagged and tied. They remove the man's cell phone from his pocket and take his gag off. The man says "bomb," and seconds later, the building explodes, trapping McGarrett and Danny inside.

Hours earlier, McGarett gets a phone call from Halawa Prison, where Jason Decker asked to see him and Danny.

Decker tells them he wants a deal: he wants to be put in a minimum security prison, and also wants his sentence cut in half. He uses important information as leverage. He tells them that a gun supplier in the Philippines is bringing an illegal gun shipment in an hour.

Back at the exploded building, McGarrett regains consciousness and looks for Danny. Danny is injured and trapped underneath rubble and large rocks.

They decide to make noise in the hopes that someone will hear them. Five-0 is on the scene looking for them, and Catherine (Michelle Borth) hears their noise-making. She calls McGarrett's cell phone, and McGarrett tells her their location. Catherine arranges a video call with them to find out their location, and captures a photo of the man who was tied up, who is now dead.

Danny finds anticoagulant pills in the deceased man's pocket in addition to a scar, meaning he most likely had surgery recently.

The Five-0 team checks local hospitals and DMVs. They find a file on Marcus Decker, Jason Decker's younger brother. They find out Decker has been working with the ATF for months, and that someone decided to kill his younger brother for revenge. Chin (Daniel Dae Kim) and Kono (Grace Park) speak to Jason, and he says that a man in his cell block named Spider gave him the intel.

Decker tells Chin and Kono that Spider paid him $5,000 and promised him $20,000 more if he would get Five-0 to the garage.

Meanwhile, at the building, Danny and McGarrett suffer another cave-in and lose cell phone reception. McGarrett then comes up with an idea to use a grenade and a pipe to blow their way out. It's successful, and the two are rescued.

Once they are outside, Chin tells McGarrett that a burner phone has been traced back to Edward Cobb, an ex-CIA agent who used to run Black Ops out of Southeast Asia in the 1970s. McGarrett then realizes that the explosion is about his mother.

McGarrett calls Cobb on the phone, then goes to his office to force him to divulge who's in the grave in Cambodia. He said it is Wo Fat's mother, who was accidentally killed when Wo Fat's father was targeted, and that Wo Fat's father is still alive.

Cobb then goes to shoot McGarrett, but McGarrett is able to shoot and kill him before Cobb is able to get his gun.

On the next episode, "Pe'epe'e Kãnaka," "As Five-0 investigate the death of a pool cleaner, they uncover a group of radicalized students who may be plotting a terrorist attack on U.S. soil."

Later, on the season finale, TVGuide reports that Grover's (Chi McBride) daughter will get kidnapped.

"As renegade and out of control as [Grover] thought McGarrett was in the [season premiere], he goes even further in this one," executive producer Peter Lenkov said. "He doesn't have a free pass like Five-0 does, so there's going to be consequences. [But] sometimes you have to turn the badge around to get the job done."

The next episode of "Hawaii Five-0" airs Friday, April 25 at 9 p.m. EDT on CBS.