```ts
export function capAfter(passage: string): string {
  if (typeof passage !== 'string') {
    throw new Error('Input must be a string');
  }

  const sentences = passage.split('. ');
  let result = '';

  for (let i = 0; i < sentences.length; i++) {
    const sentence = sentences[i];
    const firstLetter = sentence.charAt(0).toUpperCase();
    const restOfSentence = sentence.slice(1);
    result += `${firstLetter}${restOfSentence}. `;
  }

  return result.trim();
}
```