Spoiler Warning: Details of the plot are given away below.
Sakura is on the back seat of a bus, thinking back to the day before when Tomoyo asked her to come over to her place to help resolve an issue that was brought up but evaded any further inquiry regarding the subject. Just as Sakura puzzles over just what Tomoyo was referring to her bag moves and Kero pops out, causing her to nudge him back in alarmingly. She finds nearly everyone else on the bus staring with bewilderment, to which she embarrassingly dismisses any concern. She promptly chides Kero for bursting out who argues that the conditions within the bag are far too stifling for him; she continues to encourage him to keep out of sight despite his protests which still attract some attention, but fail to set any alarm bells ringing.
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After alighting from the bus Sakura wanders along a set of railings, trying to find the location indicated on the directions Tomoyo provided. Kero pops out and decides to help by going higher, assuring Sakura that no one would be able to see him at that time. Before long he announces that he's found Tomoyo's home and lifts Sakura up to see... that she's been next to it all along; Tomoyo lives in a very big house indeed.
Sakura reaches the main entrance and rings the bell, whereupon an unfamiliar voice comes over an intercom who prompts Sakura to identify herself. In short order the gate opens without visible assistance and Sakura advances, staring in wonder at the immaculate gardens. Tomoyo suddenly pops out the front door to greet her and Kero, who only pokes his head out upon hearing her voice, and leads Sakura in by the hand. The interior leaves them no less awed when a maid addresses Tomoyo with distinct formality, asking about where she'd like tea served. Tomoyo requests it in her own room but leaves the maid stumped when she asks for three cups. Sakura and Kero are both amazed by the sheer size of Tomoyo's room, which includes her own private media room for working on her videos of Sakura, who soon finds another outfit that she had been working on. As Tomoyo declares that she expects no tangible reward for making Sakura clothes a knock at the door sends Kero dashing for cover. Tomoyo's mother, Sonomi Daidouji, enters when invited and readily greets Sakura, explaining that she left her office almost as soon as she heard about her visit. She invites Sakura to partake of tea and cake in the garden, which stirs Kero's emotions, however the maid arrives with the tea just as Sonomi leads the two girls out. Once everyone's gone Kero throws a great big tantrum over his absence from the refreshments, but it's cut short when he feels something off in the area.
Outside Sonomi remarks about the expected quality of the cake and how Sakura's mother, Nadeshiko, loved sweet food just as much as Sakura appears to. When Sakura explains that her father said the same thing she unwittingly spoils the moment for her. Sakura affirms Sonomi's expectation about the cake.
Back at the Kinomoto household Touya's busy washing the car when Yukito turns up and offers to help out with the chores, which Fujitaka welcomes readily and invites him to join them in a short break. When Yukito asks where Sakura is Touya mentions that she's gone over to Tomoyo's place, which concerns him as he recalls how Sonomi reacted to Fujitaka during the sports' day at Tomoeda Elementary. However Fujitaka reassures them with total confidence that Sonomi would not aim to cause Sakura any distress.
Back at the Daidouji residence Sakura has just finished off her slice of cake when she notices that Sonomi appears to be staring into space. She mentions that Sakura reminds her of Nadeshiko in so many ways, listing off a few of her attributes before her mind wanders back to a number of cheerful moments she shared with her. This is swiftly cut-off when she recalls Nadeshiko's announcement of her engagement to Fujitaka. She then asks about Sakura's athletic pursuits, who shrugs off the remark about her skill in that field, before Sonomi mentions that is one area where she differs from her mother. She then recalls several times where Nadeshiko had a mishap, culminating in the time she met Fujitaka leading to Sonomi ranting into the sky over what he said to her at the time. She then remembers who's with her and promptly collects herself with an apology. Sakura then asks her about her father, mentioning that he never told her about himself but often talked about her mother. Sonomi says there's nothing good she can say about him, because there was nothing he could be faulted on.
Their time together is interrupted by a phone call from Sonomi's secretary, which leaves her with no alternative but to cut tea-time short. Tomoyo mentions that they'll spend time in her room and takes the last piece of cake with her; as they make their way back to the house Sonomi practically begs Sakura to join them for dinner.
Once they're back in Tomoyo's room Kero promptly chews Sakura out for leaving him out, especially since he could see how much she enjoyed it from the window. Tomoyo presents him with the last piece and he switches to great joy, Sakura's grateful to Tomoyo's foresight managing to avoid a grudge. Kero's so busy indulging in the cake that he mentions the presence of a Clow card in the manner of it being an afterthought. Tomoyo mentions that she suspected this to be the case and brings out a jewellery box, explaining that her mother wanted to have a look at the contents after an undetermined amount of time but that the box is effectively impossible to open now. When Kero points out that the card's presence is coming from the box, Sakura focuses to confirm this. Tomoyo illustrates the problem they've been having by attempting to unlock, whereupon the key shoots right out of her grasp and lands on the floor behind them. She explains the contents are family treasures in effect and that they know they're sill inside, but it's a case of so near yet so far. Sakura readily consents to providing her help in this endeavour, which leads to Tomoyo readying her camcorder.
Kero and Sakura both focus intently upon the box in an effort to find some clue as to which card's responsible and how to fix the issue, but no evidence is forthcoming. Sakura decides to give the box a tap with her staff and notices something unusual when it's deflected, a second attempt leads to Kero diagnosing the cause as the Shield card when they both see a forcefield surrounding the box. Tomoyo mentions that she couldn't see a thing, which Kero explains away by the fact that it can only be seen by those who are magic. Sakura wonders if the card's deliberately hampering Tomoyo's efforts to open it but Kero mentions that Shield's not the type to do something for kicks, it simply has a burning desire to protect whatever's valuable leading to Tomoyo smiling happily at the implication. Kero then reminds Sakura that she has a sword at her disposal that can be as sharp as she wishes, leading her to summon the power of the Sword card. When she successfully penetrates the shield the card manifests in its true form, allowing her to seal it.
A second attempt to unlock the box goes without a hitch and Tomoyo consents to Sakura's request to look at the contents herself. A knock at the door sends Kero running for cover again and Sonomi returns with an apology for cutting their time earlier short. She's surprised to find that the box is open and asks how they managed, to which Tomoyo provides a plausible explanation. When Sakura remarks upon the bouquet Sonomi takes out she mentions it was Nadeshiko's wedding bouquet, composed of cherry blossoms, she then talks about how much Nadeshiko loved cherry blossoms and recalls her mentioning that should she have a daughter she'd name her 'Sakura'.
Sonomi quickly decides that she'll personally prepare dinner and leads Sakura to the dining room, asking what she'd like the most. Once they've left Kero pops out again just as Tomoyo replaces the bouquet and takes out a small packet, affirming Kero's deduction that it's her cherished possession. He's surprised to learn that it's an eraser, but for Tomoyo it harkens back to the day she first met Sakura. Sonomi calls for Tomoyo's assistance just as she closes the box and Kero asks her to save some for him.