Note - There are a few things Regulus says and does that may baffle readers. They certainly baffled Sirius in this chapter, but like Sirius they'll get some form of answer regarding what he says and does in the next chapter. Also, Regulus theory which ends up actually working out involves something that bothered me regarding how the spell would work, yet also plays into Voldemort not really understanding how the magic he is using works.

Saving Regulus
Out of Character

The dread Sirius felt didn't go away when he stepped into the cave after Regulus. A quick look around revealed the only light sources as the tip of his brother's wand and a spout out in the middle of the cave in the distance which glowed green, yet the lights didn't cast much light. Despite the lack of light, Sirius glimpsed the water's edge which his younger brother approached while looking around for something. Regulus' fingers traced the air before shaking his head, as if unsure of something.

He watched his brother turn, looking specifically at Kreacher who also followed him into the dank place. "Kreacher, is this the place?"

"A little further to the right Master Regulus."

Sirius of course couldn't read Regulus and figure out what his younger brother planned. Instead, he watched his brother move a little to the right while Kreacher indicated Regulus was in the right place, but he muttered something Sirius couldn't hear. A green glowing chain appeared out of nowhere, soon followed by a glowing green boat. In the back of his mind, Sirius somehow knew the boat would allow them to cross the lake.

Regulus took a deep breath. "Theoretically, this should work."

"What do you mean theoretically?" Sirius shook his head, his misgivings continuing to grow.

Instead of answering the question, Regulus turned and looked at Sirius. "If I recollect, you got into trouble growing up for setting something on fire with magic."

"Oh. Thanks." Sirius knew full well he'd set some certain things on fire, much to the exasperation of his mother Walburga Black, but he didn't understand what that had to do with anything going on.

"That means you know fire spells." Regulus of course didn't pick up on the sarcasm in Sirius' voice, although this really wasn't anything new. "You'll probably need them. Actually, there is a high probability you'll need them." Sirius opened his mouth, but then Regulus said something else. "I'll get into the boat first, then you."

The feeling of dread didn't leave Sirius as Regulus stepped into the boat, remaining emotionless as ever as if they weren't in a serious situation. Hesitantly, Sirius got in himself, expecting the boat to sink, because somewhere in the back of his mind it didn't make sense for Lord Voldemort not to have spelled the boat to carry only one adult wizard or witch, but as he slid into a sitting position in the cramped boat, Regulus piped up. "Kreacher, meet us on the island."

"Yes, Master Regulus."

"By the way, don't touch the water."

This almost made Sirius touch said water accidentally as the boat took off across the body of water. "Bloody hell Regulus!"

Regulus of course looked at him in surprise like he'd seen his brother do so many times, yet this time it was accompanied with definite signs of mental strain from a lack of sleep among other things. "Didn't I tell you that this would theoretically work?"

"Regulus!" Sirius let out a huff, tucking his hands into his lap so he might not accidentally touch the lake and thus bring on whatever dangers lurked there. "Theoretically? Really? You could have forewarned me that you might not think this would have worked, couldn't you have?"

"But didn't you yourself figure out that Voldemort would have only allowed for the boat to carry just one adult wizard or witch?"

"Are you saying Voldemort was lax? Because he's the last person I'd ever think would be lax."

Regulus glanced to the side, pondering what Sirius said carefully. He then turned his head and looked at his brothers. "Not so much lax so much as not really understanding the theoretical properties behind spells which are created to purposefully focus on adult wizards."

Which of course honestly didn't explain anything at all. Sirius pinched the bridge of his nose, closing his eyes. "Reg, how many times growing up did I tell you that not everybody understands your thought process, but that some of the things you come up with don't make any sense unless you explain them?"

"But surely you of all people would know!"

Sirius opened his eye so he could peer over the hand pinching the bridge of his nose at his younger brother who seemed quite confused. "Why? Why would I of all people surely know whatever it is you're trying to tell me?"

"Because you love Muggles. That's why."

Which of course only made sense in Regulus' mind. "I don't follow."

"Because it's a theory based on Muggle science."

Sirius didn't know what was more surprising; the fact Regulus actually knew about some Wizarding theory based on something going on in the Muggle world given the way he followed their parents' ideas regarding pureblood supremacy or the fact Regulus honestly believed Sirius knew everything about Muggles.

"Scratch that. It's completely like you to actually think that my love of Muggles means I actually know everything about them, but I honestly don't. Particularly anything overly complicated." He took a deep breath. "Speaking of which, it honestly perplexes me that you would actually know about any theories which utilized knowledge of Muggles."

"Why?" Regulus of course seemed confused which left Sirius also confused, allowing his younger brother to continue as if they were having a normal everyday conversation when they weren't, which in itself was a normal thing for Regulus. "Anyways, the point is Voldemort would have overlooked said theory, right?"

"Okay." Sirius still didn't follow where his younger brother was going.

"Well, the mainstream theory behind spells which are coded around adult wizards is that these spells work based around when a wizard becomes of age or not, right?"

"Doesn't that mean that Voldemort didn't use this kind of spell?"

"No. See, that particular theory forgets that what determines whether a person is of age or not is their birthdate, which only the ministry will have records of. The vast majority in the Wizarding community don't have access to these records right?"

"Regulus, please get to where you're going with this."

"Well, the theory is based on Muggle studies which pretty much explores a child's development into adulthood and how this varies from person to person, but more importantly the theory puts forth the idea that these kinds of spells would register a person as an adult depending on their actual physical development because it's certainly not based on their weight, etcetera."

Sirius' jaw dropped, then he said, "Regulus, I'm sorry. But I am still not following you."

"Sirius, how do I look to you?"

"How old do you…" Sirius let out a soft snort. "Sixteen. You still look like a bloody sixteen-year-old."

"I was afraid you say that. No, I'm pleased you did, considering if I was wrong…"

"Wait a second." Sirius felt he better understood what Regulus meant by theoretical. "Are you telling me you had me get into the boat without knowing what would actually happen when I did?"

"I'd planned on telling Kreacher to pull you out, but you also said you know fire spells, so you should have been fine."

"And what the bloody hell about you?" Sirius felt his blood go cold. "Are you bloody suicidal Reg?"

Regulus at that moment glared at him, a look of determination on his face that made Sirius' stomach sink. "I came here completely expecting not to make it out alive Sirius."

"Bloody hell!" Sirius honestly wanted to throttle his younger brother.

"I don't understand why you're getting mad at me. You of all people know Voldemort needs to be stopped, and while this won't actually outright stop him it will hinder his progress. I brought you along because if something happens to me I need you and Kreacher to finish it."

Sirius suddenly understood where the feeling of dread stemmed from, what honestly worried him since they arrived at this place. He'd already known before Regulus said what he did that his younger brother had gotten himself in over his head, but knowing only made the feeling even worse.