'This Is Us' season 1 episode 12 spoilers: Fireman Joe and Dr. K take center stage in 'The Big Day'

"This Is Us" comes full circle as the series features the day of the big three's birth in full in the next episode aptly titled "The Big Day."

NBC's breakout hit drama-comedy series puts the present aside for a while as it delves fully into the past, giving its viewers a glimpse of the events leading up to the moments featured in the show's premiere. Series creator Dan Fogelman recently teased via Entertainment Weekly's Spoiler Room that Joe (Brian Oblak), the fireman who took Randall to the hospital, will be given his due attention as the episode follows his life in the 24 hours it takes to bring the story right up to the moment he finds a baby outside the fire station.

The episode will also feature a glimpse of Dr. Katowsky's (Gerald McRaney) life as he struggles to come to terms with the loss of his wife. Fondly known as Dr. K, Katowsky was the doctor who delivered Rebecca's (Mandy Moore) triplets, provided comfort to the couple when they lost the third baby, and in some ways, inspired Jack (Milo Ventimiglia) to adopt the baby who was to become Randall.

The fireman, Joe, will also be dealing with a conflict of his own as he fights to save a crumbling marriage. What realizations will finding the baby bring him? Will these be enough to fix his relationship with his wife?

And remember how Jack's birthday celebration seemed to have been put together in haste during the premiere episode? It turns out that a stressed and emotional Rebecca has actually forgotten her husband's birthday as she gears up for her triplets' birth.

The trailer for the next episode teases a different view of the day it all began through the Pearson couple's excitement over their babies' impending birth, Dr. K's wisdom fueled by his grief, and the personal struggles of the fireman who brought the miracle that completed the Pearson family.

"This Is Us" season 1 episode 12 airs on Tuesday, Jan. 17, at 9 p.m. EST on NBC.