The episode also has the characters deal with the consequences of Emma Swan (Jennifer Morrison) and Killian "Hook" Jones's (Colin O'Donoghue) time traveling in the third season finale.
Aware that the ship is sinking, Gerda's husband Agnarr finds her writing a letter to their daughters, Elsa and Anna, insisting that she needs to finish so that they can know the truth.
Elsa feels guilty that she may be the reason for their deaths, but Anna sets out to prove it wasn't, and meets with Grand Pabbie, the Troll King, to find out where their parents had gone.
Emma Swan (Jennifer Morrison) leaves the diner to apologize to Regina (Lana Parrilla) for breaking up her and Robin Hood (Sean Maguire) — whom she inadvertently reunited with his deceased wife Marian (Christie Laing) after traveling back in time.
She reimprisons him within the Magic Mirror, promising to release him once she accomplishes her goal, to show her the exact moment she captured Marian, only to be given a visual reminder of herself as the Evil Queen.
Grumpy tells Emma about the incident with Sleepy; she and Killian "Hook" Jones (Colin O'Donoghue) follow a trail of ice that Elsa left which leads to a warehouse.
The episode, thanks in part to the buildup around the Frozen storyline, saw its biggest numbers since the second season, as it pulled in a 3.5/11 among 18-49s with 9.47 million viewers tuning in, despite tough competition from NBC Sunday Night Football (which won the night), CBS' freshman hit Madam Secretary (which won the time period despite seeing a drop in viewers) and Fox's The Simpsons (who pulled in higher 18-49 numbers).
"[7] Amy Ratcliffe of IGN rated the episode 8 out of 10, signifying positive reviews, saying "The Season 4 premiere of Once showed the addition of Anna and Elsa works for the series, but it did more than focus on the sisters.
Regina and Rumple both took strides forward, and that sends the signal that the front half of the season won't be all Frozen all the time - unlike the trip to Neverland last year.
Instead of sprinkling on the Once twist, the portrayals of the Frozen characters were rather faithful, keeping true to the DNA of the Elsa [...], Anna [...], Kristoff [...], Grand Pabbie and even Sven that we knew on the big screen.
"[10] He then called the highlight of the episode "Lana Parrilla, who continues to pepper her Evil Queen with just the right amount of realism to make her deliciously wicked deeds seem justified, but Frozen is just the thing that has gotten Once really moving.
"[10] Brian Lowry of Variety gave the episode a generally positive review, saying ""Frozen" might not be able to glide through all the mazes that have taken "Once Upon A Time" from boundless promise to convolution, but incorporating characters from Disney's animated smash has made the ABC series feel a whole lot cooler.
The season premiere is heavily driven by the arrival of Elsa, the movie's ice queen, in a continuing plot that will have her searching for her sister Anna.
Frankly, it's still surprising the studio would risk such a formidable asset in this manner, but the stunt should help rekindle interest in a program whose happiest days appeared well behind it.