Perhaps Sirius catches Harry and Hermione as they are trying to communicate with a mysterious magical creature that they believe can help them in the fight against Voldemort. Sirius warns them of the potential dangers but is ultimately intrigued by their idea and joins them on their quest. Or it could be that Sirius catches them when they are sneaking into the restricted section of the library to find a book that might hold the key to defeating a new and powerful dark wizard, and he uses his experience to guide them through the dangerous research.
One possible plot idea could be that Sirius catches Harry and Hermione in the middle of a secret plan to save a fellow student from a dark magic threat. Sirius, initially thinking they're up to something dangerous, then realizes their noble intentions and decides to help them. Another idea is that Sirius catches them while they are exploring a hidden part of Hogwarts that is related to an old mystery about his family. He then shares his knowledge about it with them, leading to an exciting adventure.
In some fanfictions, Sirius catches Harry and Hermione as they are attempting to use a time - turning device to go back in time and prevent a tragic event that occurred in the wizarding world. Sirius, having his own experiences with time - related magic, tries to dissuade them at first but then realizes that they might be onto something important. He decides to assist them, and together they face many challenges and unforeseen consequences as they travel through time.
Chapter 653, "Everyone's Champion", is the clearest early answer because it turns Wilderness King's situation into a concrete plot event. Early on, As dawn broke, "Wilderness Survival 30 Days" released a notice on....
Chapter 687, "How Did She Die Again?", is the clearest early answer because it turns Yin Le's situation into a concrete plot event.
Chapter 691, "Sorry, You Don't Deserve It", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter gives readers a real answer by opening on As soon as Jiang Li was mentioned, Zhao Lang suddenly became...
Chapter 746, "They Are Each Other's", is the clearest early answer because it turns Other's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The fervor in that young man's eyes was something she would never...
Chapter 820, "He’s actually even harder to coax than Buchen!", is the clearest early answer because it turns Waa's situation into a concrete plot event. Early on, Today, he was particularly cautious with his eating, even sipping his....
The best chapter pointer here is Chapter 9, "The Qiao Family's business isn't big, but they have quite the audacity". It opens with Exactly, the Qiao Family isn't that great either. and closes on Qiao, unable to save face, slapped her daughter across the face, "Didn't I....
Chapter 933, "Gu Pansheng [Extra · Main Character Chapter]", is the clearest early answer because it turns Gu Pansheng's situation into a concrete plot event. Early on, Even though Mom and Dad are often not at home, we have several aunts....
If you want the clearest chapter reference, go to Chapter 3. Early on, Qiao was still annoyed at the attitude of this unknown girl when she heard...; later, the chapter leans on She's actually Miss Jiang Li from the Jiang Family?!.
Chapter 764, "Has Someone from the Rong Family Arrived?!", is the clearest early answer because it turns Rong's situation into a concrete plot event.
Chapter 884, "Master?!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter turns that into something concrete by moving from I've heard this temple is quite magical, but why are there so many...