def extract_docstring_summary(docstring):
    """Get the first summary sentence from a docstring.

    Parameters
    ----------
    docstring : `list` of `str`
        Output from `get_docstring`.

    Returns
    -------
    summary : `str`
        The plain-text summary sentence from the docstring.
    """
    summary_lines = []
    for line in docstring:
        if line == '':
            break
        else:
            summary_lines.append(line)
    return ' '.join(summary_lines)
