It is the Day of Hearts in the kingdom of Corona, which celebrates the unity between the kingdom and it's former rival, Saporia. Centuries ago the two kingdoms were engaged in a years long war. Corona's then king, Herz Der Sonne, created a complex system of tunnels beneath the kingdom to move his knights and supplies whenever he needed. The king made a detailed map of these tunnels and their secrets in a secure journal. At the height of the war, Der Soone's sworn enemy, General Shampanier, launched a full-scale invasion on Corona. Both Herz Der Sonne and Shampanier battled to neither side relenting, until Shampanier discovered that the journal's last pages contained Herz's profession of love for her. It was then realized that she held similar feelings for him as well. And soon, Herz and Shampanier were married and lasting peace was brought to the newly united nations forevermore. In honour of this romantic unity, the kingdom commemorates each year by displaying the book, so all those who have found true love may sign its pages. While everyone is in the spirit of this memorable occasion, Cassandra is feeling moody by the romantic atmosphere. This leaves Rapunzel feeling worried about her friend, that she tries to cheer her up with things like a ladies lunch, a card hidden in her hair, a portrait of Cassandra, and painting Eugene's face on her sparring dummy. All which Cassandra rejects. At last, Rapunzel expresses her concerns for Cassandra only to be told not to worry. She tells her that she is dealing with some stuff that she is not ready to share with.
By evening, while the book is being put away for the night, Rapunzel and Pascal spot Cassandra sneaking out of the castle. They follow her to the docks and discover that she has found a boyfriend named Andrew. Rapunzel and Pascal hide behind a stack of crates, being very quiet not to alert them. However, Rapunzel slips and falls into the water, alerting Cassandra and Andrew. They lift her out of the water, and Andrew gives her his jacket to keep her warm. Rapunzel is anxious to know about Andrew, only for Cassandra to get her to leave. The next day, Rapunzel remains overwhelmed with happiness for Cassandra, but Eugene shows distrust towards her boyfriend. To make things worse as well uncomfortable for Cassandra, Rapunzel invites her and Andrew on a double date. Despite Eugene and Cassandra's objections, Rapunzel manages to persuade them. During a dinner date, it is revealed that Andrew is some sort of anthropologist who came to see King Herz Der Sonne's book. He sent the Captain of the Guards many letters requesting an opportunity to study the book up close, only to have all his requests to be rejected, under security reasons. So instead, Cassandra invited him to come and see the book on Day of Hearts when it was available to the public. They wrote letters to each other and soon got to know each other, very well.
Later, Andrew takes the three of them on a balloon ride. Pretty soon, Cassandra has had enough of this double dating and asks Rapunzel nicely for her and Eugene to give her and Andrew some alone time once the balloon lands. Andrew allows Eugene to bring the balloon down, but he turns the value the wrong way which causes Andrew to lose his pendant in a briar patch below. After dropping off Rapunzel and Eugene so they can find his pendant, Andrew and Cassandra finally get some alone time. Cassandra apologises for the way her friends had been acting, only for Andrew to say not to worry. He is just glad that they finally get to be alone. As they begin to descend, Cassandra and Andrew nearly share a kiss until the balloon gives them a jerk after touching the ground. After stepping out of the basket, Cassandra invites Andrew get it a private look at the book so they can write their names in. Andrew is delighted to, and being the Captain of the Guard's daughter, Cassandra has the keys to see it. Meanwhile, after finding Andrew's pedant broken, Rapunzel and Eugene take it to Xavier's to be fixed. While mending it, they learn from Xavier that the emblem in the pendant is the symbol of Separatists of Saporia. According to Xavier, after the war brought Saporia and Corona together, there was a small sect of Saporians who felt betrayed, and swore revenge on the kingdom. It has been rumoured that the descendants of the Saporians are still around, waiting for the perfect opportunity to strike back at Corona. Upon realizing who Andrew actually is, Rapunzel and Eugene rush back to the castle to save Cassandra. Meanwhile, Cassandra sneaks Andrew into the room where the book is kept, and as Cassandra begins writing in their names Andrew quietly pulls out a sword. However, just as he is about to strike her down, Cassandra unexpectedly disarms him and has him at her mercy. It is revealed that Cassandra already knew he was a fraud and had been playing along with him since day one. However, just as she is about have him cuffed, Rapunzel barges in to warn her. Andrew, really known as Hubert, disarms Cassandra and cuffs both her and Rapunzel to the podium before escaping with the book, planning to deliver it to his fellow Saporians for their plans against Corona.
While cuffed to the podium, Cassandra starts scolding Rapunzel for messing up her plans. Rapunzel also scolds Cassandra for not telling her the truth, only for her to say that she could not risk her letting anything slip. Proving her point that Rapunzel cannot keep secrets, Cassandra brings up the fact that she told Eugene about the night they snuck out, telling her father about the rocks she showed her, and telling her mother who ate the last chocolate truffle. Cassandra respects that Rapunzel is a sharer, but tells her she has to respect that there are some things she would like to keep to herself, even if those things have life threatening consequences. At last Rapunzel understands, and promises to respect Cassandra's privacy from now on if she understands that if something is bothering her she will want to help. Just then, Eugene arrives and after freeing Rapunzel and Cassandra the three of them go after Hubert.
They arrive just in time as Hubert takes off in his balloon. Cassandra manages to get aboard and engages Hubert in a sword fight. Eugene slightly manages to get aboard, and Rapunzel tries to use her hair to stop the balloon. Unfortunately, she fails and ends hanging from her hair as the balloon floats over the forest out of control. As the battle wages on, the Balloon gets wrecked, and does not come to an end until Cassandra takes out Hubert with a single punch. Afterwards, Herz Der Sonne's book is safely returned, Hubert is arrested, and Cassandra is made an honorary hero.
- Mandy Moore as Rapunzel
- Zachary Levi as Flynn Rider
- Eden Espinosa as Cassandra
- Dean Winters as Andrew
- Adewale Akinnuoye-Agbaje as Xavier
- Jeffrey Tambor as Big Nose
- Paul F. Tompkins as Shorty
- Steve Blum as Attila
- Diedrich Bader as Stan
- The character Xavier is a deleted character from earlier versions of Disney's 50th animated feature, Tangled.
- The look of King Herz Der Sonne and General Shampanier are based off early designs of King Frederic and Queen Arianna.