NBC is treating its loyal TV viewers to an epic three-way, two-day crossover event among "Chicago Fire," "Chicago Med" and "Chicago P.D." The various public servants of Chicago will band together to save one of their own, while uncovering a dangerous mystery along the way.

The Chicago crossover will start in the midseason premiere of "Chicago Fire." According to a synopsis for episode 10 posted on Carter Matt, the members of Firehouse 51 descend on Chicago Med and wait for news about Herrmann (David Eigenberg), who was stabbed by Freddie (Ralph Rodriguez) in the winter finale. Meanwhile, Cruz (Joe Minoso) works with Chicago P.D. officers in tracking down the missing Freddie. Dr. Choi (Brian Tee) and Dr. Halstead (Nick Gehlfuss) treat a patient who attempted suicide but whose condition raises some alarms.

The trailer for "Chicago Fire's" season 4 midseason premiere, titled "The Beating Heart," begins with Herrmann being discovered by Dawson (Monica Raymund) and Otis (Yuri Sardarov) lying in a pool of his own blood in Molly's storeroom. An accompanying voiceover introduces the two part crossover between all three Chicago teams – "Fire," "P.D." and "Med."

This is followed by the members of the Firehouse 51 team arriving at the hospital while doctors from "Chicago Med" work on Herrmann. A mystery is reportedly unearthed in the midst of a crisis as a doctor says a female patient is suffering from an overdose of chemotherapy. However, there are no records that the patient has cancer. Boden (Eamonn Walker) is seen trying to raise Casey (Jesse Spencer) on the horn when a house explodes and is engulfed in flames. The clip ends with the fire chief shouting for Casey.

Herrmann's condition seems critical but he's in good hands as Dr. Connor Rhodes (Colin Donnell) and Nurse April (Yaya DaCosta) do their best to save the fire fighter. But the synopsis for episode 5 of "Chicago Med's" first season hints that tension will develop between one member of 51 and Dr. Rhodes when it was reported that several doctors felt that Herrmann should've been operated on sooner.

A conspiracy is also revealed when members of "Chicago Fire" save a woman from a burning building. The woman, Jessica, is rushed to the hospital where doctors initially thought she was suffering from cancer. But it's later revealed that she doesn't have cancer but was being poisoned by unnecessary chemotherapy.

This conspiracy will play out in part two of the crossover as Voight (Jason Beghe) discovers that one doctor has been poisoning patients via unnecessary chemotherapy.

The crossover between the three Chicago-based shows will kick off on Jan. 5 and will continue the following day.