```python
def cap_after(passage: str) -> str:
    """
    Return a passage with the first letter of every sentence in capitals,
    a sentence beginning at the start of the passage and again after any full stop.
    Letters anywhere else are left exactly as they are.
    """
    if not passage:
        return ""

    result = passage[0].upper()
    for i in range(1, len(passage)):
        if passage[i] == '.' or passage[i] == '?' or passage[i] == '!':
            result += ' '
            result += passage[i + 1].upper()
            i += 2
        else:
            result += passage[i]

    return result
```